Chromizing with improved utilization of gas



Nov. 27, 1962 G. A. SAMUEL,

CHROMIZING WITH IMPROVED UTILIZATION OF GAS Filed Feb. 19, 1959 INVENTOR George A, Jam/wed ATTORNEYS.

United States Patent Office fitififl i l Fatented Nov. 27, 1962 3 065,944 CHROMIZHNG WTTHEMPROVED UTELHZATEEPN F GAS George A. Samuel, White Plains, N.Y., assignor to Alloy Surfaces @ompany, Inc., Wilmington, DelL, a corporation of Delaware Filed Feb. 19, 195%, Ser. No. 794,335 4 Qlairns. (Cl. 117107) by facilitating the regeneration of the chromizing gases by causing the chromium to drop through the chrornizing gases and exposing a maximum surface area of chromium to the gases.

A further purpose is to reduce or eliminate the sintering of chromium particles on the Work.

A further purpose is to provide vanes inside the chromizing retort which will carry chromizing particles up and drop them through the gases and on to the work.

A further purpose is to place the work in a perforated container, and to place the vanes on the outside of the container. 1

A further purpose is to place the work in a perforated container and to place the vanes on the inside of the retort around the container.

Further purposes appear in the specification and in the claims.

FIGURE 1 is a diagramamtic sectional perspective of a retort for chromizing according to the invention.

FIGURE 2 is a fragmentary enlarged diagramatic sectional perspective of the retort itself and the container for the work in the preferred embodiment, the section being taken on the line 2--2 of FIGURE 1.

FIGURE 3 is a perspective of the container for the work removed from the retort of FIGURES 1 and 2.

FIGURE 4 is a transverse section of the retort and container for the work showing a variation.

In the prior art practice in rotary chromizing it has frequently been preferable to use the chromium (or ferrochrome) in the form of lumps ranging from to to /2" in size. This gives excellent results, but the lumps occupy a relatively large proportion of the space in the retort, leaving less space for work.

Since chromizing depends upon relative surface of the work and the chromium particles, it has been normal practice to use chromium (ferrochrome) two or three times the weight of the work. The void space between the lumps helps in circulation of gas, but also consumes a considerable amount of volume.

If, instead of chromium (ferrochrome) lumps, chromium (ferrochrome) powder is used, suitably through 40 mesh and preferably through 109 mesh, the surface area of the chromium or ferrochrome is so greatly increased that the total weight of chromium particles can be considerably reduced.

Attempts to use fine chromium or ferrochrome powder in rotary chromizing have not, however, met with great success. One of the primary difficulties is that the powder has tended to sinter on the work. Also the rate of chromizing has not been as good as the available surface area of chromium would lead one to expect. It appears from my experiments that a considerable part of the difiiculty is due to the fact that particles of chromium accumulate at the bottom of the retort and under the pressure imparted by the weight of the work located above the particles of chromium sinter to themselves and to work pieces imbedded in them. Due to this agglomeration of the chromium particles, a relatively small area of chromium is exposed to the gases. Poor circulation takes place and a large portion of the chromium simply slides to the bottom of the retort.

I have discovered that this difficulty can be largely overcome by providing vanes or blades inside the retort which carry the chromium particles up as the retort rotates and deposit them on top of the work. This prevents an accumulation of a large mass of particles of chromium at the bottom of the retort where they would be subject to pressure from the mass of the Work, and also where they they would be inclined to sinter to the work. The action of the blades involves sifting of the particles of chromium through the Work and down through the gases, thus greatly improving the exposure of the particles to the gases and reducing the tendency to sinter chromium on the work.

Since the chromium never collects as a large mass, there is much less tendency than in the prior art to obstruct regeneration of the chromizing gases during chr0- mizing.

For best results the work is surrounded by a perforated container, and the particles of chromium drop through the perforations and through the work in the container.

in the preferred embodiment of the invention, the vanes are secured to the outside of the perforated container which is free to turn relative to the retort. Less desirably the vanes may be secured to the inside of the retort.

According to the invention, the whole surface of each particle of chromium or ferrochrome is exposed to gases for reaction in order to regenerate chromizing gases. This exposure is most effective when the particles of chromium are dropping through the gases. The circulation of the particles of chromium tends to make each particle of chromium take an active part in the chromizing, and avoids the formation of agglomerates of chromium particles. The fact that no particle for any prolonged time remains in contact with the work in itself reduces the possibility of sintering.

When reference is made herein to particles of chromium, it is intended to include ferrochrome which should contain at least 30 percent chromium by weight and preferably at least 63 to 65 percent chromium by weight. It is preferred to use the low carbon grade containing not in excess of 0.01 percent carbon by weight in the ferrochrome.

In the preferred embodiment of the invention as shown FIGURES 1' to 3, a suitable metallic retort 30 preferably of heat resisting alloy, has cylindrical or tubular side walls 31, and closed rear end 32 and a threaded removable head 33. The major axis of the cylinder of the retort is desirably located horizontally or generally horizontally, so that vanes moving in the direction of motion of the walls of the retort will be effective in circulating particles of chromium.

Any suitable seal can be used on the head 33 as desired.

Centrally extending from the rear end of the retort is a support rod 34 which is journalled on a bearing 35 and at the end beyond the bearing carries a pulley 36 which is driven by a suitable belt 37 to rotate the retort.

At the forward end the head 33 has extending therefrom a central tube 38 communicating with the interior, and which is supported on a bearing 40 at the front. This tube thus supports the front of the retort. The retort proper is surrounded by a furnace 41 which is suitably a gas-fired furnace or an electric furnace as desired, capable of maintaining the retort at a temperature between 1600 and 2300 F. as desired.

Beyond the furnace the tube 38 is connected to a suitable rubber or other flexible tube 42 which extends beneath the level of water or other sealing liquid 43 in a receptacle 44. As glass tube may be connected to the end of the rubber tube for inspection purposes if desired. A valve 45 is interposed to close the tube 42 when desired.

In the preferred embodiment as shown in FIGURES 1 to 3, a tubular or cylindrical perforated metal container 46 is provided within the retort, having the major axis parallel to the major axis of the retort and moving freely as the retort turns. The container 46 has perforated side walls 47 and a preferably perforated permanent closure 48 at one end and an end opening 50 at the other end which is closed by a perforated cover 51 held in place as by wire loops 52. Perforations in the metal side and preferably also the end walls of the container 46 are larger than the particle size of the chromium or ferrochrome, so that if the chromium or ferrochrome is of granular form these will be quite large, but if the chromium or ferrochrome is of fine powder form, say through 40 mesh or preferably through 100 mesh or 200 mesh per linear inch, the perforations in the container may suitably be finer, more of the character of insect screening. In any case the perforations in the container are small enough so that the steel parts constituting the Work cannot escape from the container.

Running longitudinally along the outside of the container 46 and suitably welded to it are metallic vanes 53 at suitable intervals around the circumference, in the example shown, 3. The vanes are desirably made by cutting metallic tubing longitudinally so that they form hollow recesses or pockets 54. To strengthen the structure and protect the vanes against damage, rims 55 of metallic sheet are Welded to the vanes and to the container body at the ends of the container and these rims ride on the inside of the retort, the outer edges of the vanes being flush with the outer diameter of the rims.

In operation of the retort of FIGURES 1 to 3, the container 46 is opened and filled with steel parts to be chromized which may be low carbon steel like AISI 1005 or 1010 or intermediate carbon steel like AISI 1035 or 1045 or high carbon steel like AISI 1070 or 1090. The work may also be alloy steel such as AISI 4130, 4330, or 8630. High carbon high chrome steel such as AISI 440 C may be used.

When placing the work in the container 46, it is desirable also to place the chromium particles in the container with the work and also the activator. If desired, however, the chromium particles and the activator can be placed in the retort outside the container 46.

The quantity of chromium particles will vary but it will be understood that if the chromium particles are granules of the order of of an inch of /1 of an inch the Weight of chromium particles will be of the order of two or three times the Weight of the work, while if the chromium particles are in the form of fine powder the quantity of chromium can be reduced until it is of the order of only 50 percent the weight of the work.

The quantity of activator should vary between 0.01 and 2 percent by weight of the charge work plus chromium. For a retort having an internal diameter of about four inches and a length of about twenty-four inches thirty grams of activator has often been used. The retort and the container should be dry and free from contamination.

Any one of a wide variety of activators may be used, the preferred activators being chromium chloride, ammonium chloride, ammonium bromide, ammonium fluoride, ammonium bifluoride and iron chloride. If an activator is used which does not give off a purging gas the retort should be purged as by passing chlorine, hydrogen chloride or other suitable gas through the retort to exclude air.

After the materials to go in the container 46 have been inserted, the closure 51 is aifixed and the container is placed in the retort. If the particles of chromium and the activator have not been included in the container they are placed in the retort outside the container. The retort is then sealed and the rubber tube extended beneath the water, the valve being opened. The retort is rotated and heating started. The rotation of the retort may be of the order of one half to ten r.p.m., although greater or lesser speeds may be used if desired. The retort is heated up to a temperature within the chromizing range of 1650 to 2400 degrees F. The activator, if, for example, ammonium chloride has been used, breaks down and a considerable amount of gas is evolved bubbling through the water seal. At some time in the chromizing operation, the retort may exhibit a tendency to suck in water and this can be observed if a glass tube is used on the end of the rubber tube. If this tendency becomes pronounced the valve in the tube should be closed.

As the chromizing retort rotates, the particles of ferrochrome are scooped up by the vanes or blades andcarried along the wall of the retort until they reach an upper position. Chrome powder is shown in the vanes at 56. From the upper position the chromium powder is deposited on the top of the perforated container and it Sifts down through the gases which are formed from the activator, thus tending to form more chromium containing gases from the reaction with the halogen. At the same time chromium powder remains close to the work although the particles in this position are constantly changing.

In FIGURE 4 I illustrate a modification in which vanes 57 extend inwardly from the Walls of the retort and extend longitudinally of the retort parallel to the axis. The container 46' is simply a perforated and cylindrical container with end closures and without vanes attached to it and the container is free to move with respect to the retort. As in the form of FIGURES l to 3, chromium powder accumulates ahead of the vanes at 56 and is deposited on to the top of the perforated container.

The chromizing operation according to the invention will continue for a suitable length of time, which may vary from one half hour to ten hours and in individual cases may be as long as 24 or 48 hours.

Example 1 Using the mechanism of FIGURES 1 to 3, 1500 grams of AISI 1070, saw-chain links were placed in the perforated container with 1400 grams of ferrochrome containing 63 to 65 percent chromium by weight through mesh per linear inch and 30 grams of ammonium chloride.

The retort was rotated at 3 rpm. and the furnace was started and the retort heated to 1900 F. Chromizing at 1900 F. was continued for two hours. Considerable gas was evolved during the first half hour of heating and subsequently the retort operated under a vacuum and the valve was closed.

The resulting chromized Work had a case thickness of 0.001". The case resisted boiling 20 percent nitric acid and was hard and scratched glass. The case was clean and bright and there was no evidence of sintering of chromium particles on the surface.

Similar results are obtained using chromium powder instead of ferrochrome and also using other activators such as ammonium bromide or ammonium fluoride or bifiuoride.

Example 2 Using the mechanism of FIGURES 1 to 3, nine pieces of AISI 1040 conveyor parts weighing 2 pounds and two pieces of AISI 1070 saw-chain links were placed in the perforated container with two pounds of ferrochrome containing 63 to 65 percent of chromium through a 100 mesh per linear inch and 30 grams of ammonium chloride. The procedure followed was the same as that of Example 1.

The case varied in thickness between 0.001 and 0.0015 inch and was hard and cut glass. All surfaces were clean, bright and free from sintered particles of chromium. The presence of steels of two different carbon contents did not interfere with the chromizing of any of the parts.

Example 3 Assorted machine screws including inch--20 to a total weight of 2% pounds were placed in the perforated container of FIGURES 1 to 3. The analysis was A181 1020. Also three pounds of ferrochrome containing 63 to 65 percent by weight of chromium was added in the form of of an inch particles and 30 grams of ammonium chloride. The procedure of Example 1 was used except that the retort was heated to 2100 F. and maintained at that temperature for three hours. Gas was evolved until the last half hour when there was a tendency to form a vacuum and the valve was closed.

A good case was obtained on all parts penetrating 1 deeply even into the fine threads and the nuts ran on the bolts easily after chromizing. There was no sintering of chromium particles. Excellent throwing power was exhlhited.

Example 4 The procedure of Example 3 was carried out except that the retort was not rotated and the ferrochrome was through 30 mesh per linear inch. 15 grams of ammonium chloride activator was used. The work comprised 5 pieces of A181 1010 sheet /2 x 4 x 0.034 inches and 2 pieces of A inch-20 screws with nuts.

Chromizing was carried out 2100 F. for two hours. The last half hour the operation was under vacuum and the valve was closed.

All of the parts were heavily sintered with chromium powder. The case was 0.002 inch. The nuts would not move on the threads. I

Example 5 The mechanism of FIGURE 4 was used. The work consisted of eight A181 1040 steel conveyor parts and four pieces of AISI 1010 steel sheet. Three pounds of 63 to 65 percent ferrochrome through 30 mesh was employed along with 30 grams of ammonium chloride. The retort rotated at 3 rpm. Chromizing was carried out at 2100 F. for two hours. The case was dull and silvery, thin and somewhat brittle and there was some sintering of chromium powder on the parts but nothing comparable to that encountered in Example 4.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art,

' to obtain all or part of the benefits of my invention without copying the method and apparatus shown, and I therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claim.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. The process of chromizing ferrous metal parts, which comprises placing the ferrous metal parts in a retort in the presence of particles of chromium through 40 mesh and in the presence of a halogen-containing gas, the retort being sealed against entrance of air while having an escape port, maintaining the retort and its contents at a temperature between 1600 and 2300 degrees F., relatively physically manipulating the particles of chromium with respect to the halogen-containing gas so that the particles of chromium drop through the halogen-containing gas and thereby regenerating the chromium in the halogencontaining gas and then bringing the halogen-containing gas containing chromium into contact with the work, and bringing the chromium particles into contact with the work.

2. The process of chromizing ferrous metal parts, which comprises placing the ferrous metal parts in a perforated tubular container having vanes on the outside, placing the container with the ferrous metal parts in a retort in the presence of chromium particles through 40 mesh which can pass through the perforations and in the presence of a halogen-containing gas, the retort being sealed against entrance of air while leaving an escape port, maintaining the retort and its contents at a temperature between 1600 and 2300 degrees F., rotating the retort, and by the rotation of the retort turning the perforated container with respect to the retort and by the vanes on the container raising the chromium particles in the retort, dropping them through the gases in the retort to regenerate the chromium in the gases and dropping the chromium particles through the perforations into contact with the ferrous metal parts, whereby the chromium particles make repeated cycles through the gases.

3. The process of chromizing ferrous metal parts, which comprises placing the ferrous metal parts in a perforated container, placing the container in a tubular retort having vanes on the inside of the retort along with particles of chromium through 40 mesh which can pass through the perforations in the perforated container and along with halogen-containing gas, the retort being sealed against entrance of air while leaving an escape port, maintaining the retort and its contents at a temperature between 1600 and 2300 degrees F., rotating the retort and thereby causing the perforated container to turn with respect to the retort and by the vanes on the inside of the retort carrying chromium powder up in the retort and dropping it on the perforated container, the particles of chromium making repeated cycles relative to the gases so as to regenerate the chromium in the gases, and the particles of chromium coming into contact with the ferrous metal parts.

4. The process of chromizing ferrous metal parts, which comprises placing the ferrous metal parts in a retort in the presence of particles of chromium in contact with the ferrous metal parts, and in the presence of a halogencontaining gas, the retort being sealed against entrance of air while having an escape port, maintaining the retort and its contents at a temperature of between 1600 and 2-300 degrees F., and repeatedly as an endless series of cycles bringing the particles of chromium into a relation with other particles of chromium where they are not resting on one another but are free from one another and dropping repeatedly through the gas, and where they move relative to the halogen-containing gas completely surrounded by the halogen-containing gas so as to regenerate the chromium in the halogen-containing gas, and then bringing the halogen-containing gas containing chromium into chromizing contact with the ferrous metal parts.

References Cited in the file of this patent UNITED STATES PATENTS 1,853,369 Marshall Apr. 12, 1932 1,920,678 Cowper-Coles Aug. 1, 1933 2,257,668 Becker et a1 Sept. 30, 1941 FOREIGN PATENTS 693,292 Great Britain June 24, 1953 

1. THE PROCESS OF CHROMIZING FERROUS METAL PARTS, WHICH COMPRISES PLACING THE FERROUSD METAL PARTS IN A RETORT IN THE PRESENCE OF PARTICLES OF CHROMIUM THROUGH 40 MESH AND IN THE PRESENCE OF A HALOGEN-CONTAINING GAS, THE RETORT BEING SEALED AGAINST ENTRANCE OF AIR WHILE HAVING AN ESCAPE PORT, MAINTAINING THE RETORT AND ITS CONTENTS AT A TEMPERATURE BETWEEN 1600 AND 2300 DEGREES F., RELATIVELY PHYSICALLY MANIPULATING THE PARTICLES OF CHROMIUM WITH RESPECT 